The Moroccan champions made the most of home advantage in the opening leg at the Stade Saniat Rmel, as Iajour struck three times in the first half to help his side into a healthy lead in the tie.
Morocco international Iajour needed just 14 minutes to open his account for the competition and doubled his tally before the half-hour mark.
The 29-year-old ensured his hat-trick 10 minutes prior the interval with Zaid Krouch completing the rout with a late strike.
The second leg will be played at the Sani Abacha Stadium in Nigeria on April 4.
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